1. Field of the Invention
This invention relates to a thermal printer, and more particularly, to such a printer in which lasers are used to provide the energy necessary to effect dye transfer in a thermal medium.
2. Description of the Prior Art
In one type of thermal printer, a dye-donor element is placed over a dye-receiving element, and the superposed elements are supported for cooperation with a print head having a plurality of individual heating resistors. When a particular heating resistor is energized, it causes dye from the donor to transfer to the receiver. The density or darkness of the printed color dye is a function of the energy delivered from the heating element to the donor. One of the problems in printers of this type is that the thermal time constant of the resistors is quite long. As a result, the printing speed is relatively slow, and the image contrast is limited.
It is known in thermal printing to use lasers instead of resistors to effect the transfer of dye from a donor element to a receiver element. In U.S. Pat. No. 4,804,975, for example, there is shown thermal dye transfer apparatus which comprises an array of diode lasers which can be selectively actuated to direct radiation onto a dye-carrying donor. Radiation from the diode lasers is modulated in accordance with an information signal to form an image on a thermal print medium. The diode laser array extends the full width of the print medium. One problem with this apparatus is that it is too expensive for many applications. The initial cost of such a large array is relatively high, and failure of only one diode laser in the array will result in discarding the entire array. A further problem with the patented apparatus is that it is difficult to obtain the density and resolution needed for a high-quality image.
When a diode laser is used for high-resolution thermal printing, the output from the laser diode must be focused to a small spot. As noted in the aforementioned U.S. patent application, it has been found that a single transverse mode coherent laser, having a Gaussian type output, can be used to produce a diffraction limited spot on the thermal medium. One problem of using such a laser in some applications is that the lasers emit at relative low power, and thus, the writing speed of the printer is limited.